Porous ceramics have a variety of applications in filters, sorbents, catalysis, gas separation membranes and fuel cells. By virtually controlling the composition and microstructure (porosity, pore morphology, pore size and distribution), a porous ceramic can exhibit very different physical and chemical properties to meet specific applications. To produce these application-specific porous ceramics, adequate fabrication techniques are required. A traditional way to make a porous ceramic is to mix the functional ceramic material with a fugitive pore former. When such a mixture is fired at elevated temperatures, the pore former is thermally decomposed into vapor phase, leaving behind pores within the original structure. This method, however, often results in non-uniform pore distribution due to uncontrollable grain growth in the course of sintering. In addition, uneven shrinkage caused by inhomogeneous distribution of pore formers lead to dimensional instability. It is particularly detrimental to multilayer ceramic structures such as solid oxide fuel cells when thin-films of large aspect ratio are present. To meet the increasing demand for porous ceramics with a controllable microstructure, numerous novel processes have been developed, representatives of which include methods of replica, direct foaming, and sacrificial template.
The “replica” technique is a relatively easy and well-established process; but it often produces products with degrading mechanical strength due to the cracked struts formed during the process. Since the pore sizes in the final ceramic parts are not directly related to the “pore formers” in the simple manner, the “direct foaming” method has the difficulty to control porosity. In contrast, the sacrificial template method involves sintering the matrix material with the sacrificial material at elevated temperatures and chemically removing the sacrificial material at low temperature (often at room temperature). Since the creation of porosity takes place at low temperature, the dimensional stability of the matrix can be ensured. More importantly, the porosity can be controlled by the volume fraction of the sacrificial template while the pore size and distribution can be tailored by the grain size and distribution of the sacrificial template material. However, one important requirement for this method is that the functional matrix material does not react with the sacrificial material at elevated temperatures.
While the porosity of the matrix can be easily controlled by the volume fraction of the sacrificial material, a homogeneous distribution of pores as well as pore size also requires special preparation methods. Wet-chemical routes are well known to produce fine particles with homogenous distribution. Methods such as modified pechini, combustion, hydrothermal, and co-precipitation are the most commonly practiced ones. Among these methods, the co-precipitation method is advantageous in that the co-precipitate has not only smaller particle sizes but also higher degree of homogeneity.
Nonetheless, alternative methodologies for tailoring properties of a porous ceramic for particular applications in catalysis, fuel cells and gas separation membranes would be desirable.